Hello Jenn,
I don't believe there is anything you can do. You don't say what the item is, and "tweeked" is a rather useless word. No matter, what you seem to be intimating is where your might stand on a matter of copyright, - and unless you have a clearly patented design and construction, neither ebay nor anyone else will do a thing.
Obviously ebay cannot sanction a seller on your say-so. For all ebay knows, the other seller made it first and it was you who copied. Ebay is not going to consider delving into a copyright issue for which there is no patent and hence no issue.
In matters of artistic copyright you would have to prove that the design was original enough to not be commonplace, such that no other person could possibly have simultaneously created a like image. Copyright law is not the same as 'passing off', - whereby the other seller is alleged to be pretending the work is yours. That seems not to be the case. Moreover, both works would have to be examined to determine if one has clearly and deliberately been derived from the other, -- has the other seller demonstrably copied a significant enough portion of the original work. This is not to say that the other seller would have copied a larger part of the earlier piece but what courts would deem indicative elements of 'quality', - not quantity.
What you would have to do then is prove that your work came first in time, - not that it was listed first on ebay. Otherwise it is as likely the other person designed and published it first on his website and you copied it from there. So you'd need to show yours existed first. You would then have to prove that a substantial part of your work was taken by the new one and that this could not be the result of commonplace imagery. After all, it is not an infringement to borrow an idea and transform that idea into a new work. Hence, even if copying can be evidenced it is quite another matter to prove copyright infringement and appropriation of your skill and labour. How far are you prepared to go?
You see, ebay cannot simply remove another seller or his wares because someone else claims to have made something first. Otherwise, every jealous seller would abuse the practice to remove any and all competition. If, however, you return with a court order, ebay may pay attention to that.
